Siloxane polymers and copolymers containing the trifluoropropyl group are the most common commercially available fluorosilicone polymers. Typical fluorosilicone copolymers have the general formula:MDaDFbMwith
M=R1R2R3SiO1/2;
D=R4R5SiO2/2; and
DF=R6(CH2CH2CF3)SiO2/2;
where the subscripts a and b are non-zero and positive and satisfy the following relationship: b is less than or equal to 0.4(a+b) and R1, R2, R3, R4, R5, and R6 maybe any monovalent hydrocarbon radical: alkyl, aryl, or alkaryl but are typically methyl (CH3), and in some cases can be typically vinyl. Equilibrium considerations imposes a practical upper limit of 40 mole percent on the number of D units substituted with the trifluoropropyl subsitutent. The following polymer:MDFbMcannot be prepared by equilibration reactions when b is large because, at equilibrium, cyclic silicones are the thermodynamically favored species and therefore the yield of polymer is low. Thus, when b>0.4(a+b), polymer yields are low. Because fluorosilicones possess desirable properties such as solvent resistance, higher mole percent substitution of the silicone polymer chain with trifluoropropyl substituents and polymers (and copolymers) where b is large is desirable.
Preparing liquid injection moldable fluorosilicone polymers from addition curable precursors requires either a hydride fluorosilicone, a vinyl endstopped fluorosilicone or both as addition curable components. Preparing low viscosity liquid materials that cure to a conformal coating or encapsulant from additional curable precursors also requires a hydride and a vinyl endstopped fluorosilicone as an addition curable component. A synthetically convenient route to obtaining addition curable fluorosilicone polymers has been to use the classical approach to the problem of obtaining a vinyl endstopped fluorosilicone by first making a silanol endstopped fluorosilicone by polymerizing the so-called fluoro trimer, e.g.((CH3)(CH2CH2CF3)SiO)3 using a mild non-equilibrating catalyst such as NH4OH with water as the chainstopper at high pressure, or temperatures in the range of 100-135° C. at atmospheric pressure conditions employing NaOH as a catalyst or employing KOH as a catalyst at temperatures of 50-100° C. In siloxanes polymerizations, KOH is a stronger polymerization catalyst that NaOH and will initiate polymerizations at lower temperatures than NaOH. But, even at temperatures as low as 50° C., KOH may catalyze undesirable condensation reactions of silanol terminated polymers and/or causing equilibration to occur, resulting poor viscosity control and reduced polymer yields. Typically, the silanol terminated polymers so formed are reacted with divinyltetramethyldisilazane to produce a vinyl terminated fluorosilcone polymer. It is known that other materials that can convert a silanol into an alkenyldialkyl siloxy endgroup are also acceptable for treating such silanol stopped polymers. Such material would include various alkenyldialkylamino silanes, and the like. However, such materials are much higher in cost than divinyltetramethyldisilazane, which is commercially available. This approach to synthesizing a vinyl stopped fluorosilicone suffers from the drawback that the trimer polymerization reaction with water or diols is not controllable in terms of the viscosity (or molecular weight) of the resulting silanol stopped fluorosilicone. Reaction with divinyltetramethyldisilazane only converts the molecules to the desired vinyl stopped fluorosilicone polymers adding nothing by way of molecular weight or viscosity control to the product. Viscosity control is very important for commercial products. A lack of viscosity control can cause a variety of problems. Polymer viscosity can control both physical and application properties. For example, if polymer viscosity is poorly controlled, multiple batches must be produced and blended to target viscosities. This results in excess inventories and disruption of production schedules. Further, polymer blending must be within certain ranges. Blending batches over wider viscosity ranges will change final product properties. Achieving excellent viscosity control over such polymers permits efficient production and consistent quality.
High viscosity flurosilicone rubber compounds are made by first producing a high viscosity fluorosilicone polymer, typically in a doughmixer because of the high viscosity of such polymers. The polymers are removed from the polymerizing doughmixer and transferred to a second mixing machine, often another doughmixer, where other ingredients, such as fumed silica are added. When high viscosity fluorosilicone polymers are made, they have been made by polymerizing fluorosilicone trimer at 120-130° C. with NaOH. These conditions are non-equilibrating and result in 99-100% conversion of the cyclic trimer to polymer. Thus, suitable polymer is already in the mixer for directly making the fluorosilicone rubber compounds by adding filler and other ingredients. However, after the fluorosilicone rubber compound is removed from the mixer, there will always be small amounts of such compounds left in the mixer. When it is attempted to make a second batch of fluorosilicone polymer following the production of a fluorosilicone rubber compound, the silica filler in the residual compound reacts with the NaOH at the polymerization conditions, deactivating the catalyst. This can be overcome by using large amounts of NaOH, but such larger amounts of NaOH will result in undesirable properties of the final rubber product, which is often used in extreme applications.
The equilibration polymerization of dimethylsilicones and their copolymers, from, for example, the cyclic tetramer, cyclic pentamer, or hydrolyzate, will typically produce a product with 85% polymer and 15% cyclics at equilibrium, and these polymerizations, especially to produce high molecular weight polymers used in silicone rubber are done at temperatures above 140° C. using KOH as the equilibration catalyst. Such polymers are thereafter compounded with silica fillers, especially fumed silica, and often in “doughmixers” to produce silicone rubber. The technology to do polymerization and compounding in a single step in the same mixer has never been effective because the presence of 15% cyclics at the end of polymerization would require a long and expensive stripping step, this is further complicated by the fact that at temperatures above 140° C., the KOH reacts with the silica to produce potassium silicate destroying the catalyst.